Chemistry and inherent viscosity of glasses segregated at grain boundaries of silicon nitride and silicon carbide ceramics

Electron microscopy/microanalysis and internal friction characterizations have been concurrently used to quantitatively assess both the morphology/chemistry and the inherent viscosity of residual high-SiO2 glasses segregated to grain boundaries of polycrystalline Si3N4 and SiC ceramics. The anelastic relaxation peak of internal friction, arising from the viscous slip along grain boundaries wetted by glass, was collected in a plot of internal friction vs temperature. The peak was analyzed with respect to its shift upon changing the oscillation frequency, in order to assess the inherent viscosity of the intergranular glass film. From the internal friction analysis and the precise information of both thickness and chemistry of the grain-boundary film obtained by electron microscopy, a quantitative determination of the inter granular viscosity and the activation energy for viscous flow was attempted. The viscosity of intergranular high-SiO2 glasses as a function of temperature is compared to the literature data for bulk SiO2 glass and discussed in terms of the respective anion structure of the glass.